Physically Active Medium

When plastics are brought in contact with physically active media, the latter can penetrate into the polymer microstructure as well as into microscopic defects or sink holes and reduce the intermolecular bond forces, thus increasing macro-molecular mobility. The consequences include swelling, reduction in the modulus of elasticity, glass transition temperature, strength and hardness, influence on electrical properties, changes in dimensions and discoloration. These processes are of a physical nature and mostly reversible, unless polymer components have been extracted and the structure has changed, e.g., because of secondary crystallization [502]. 

We differentiate between swelling with and without solvation that can also be accompanied by chemical attack. In non-solvating media (e.g., water), the result is an accumulation of media molecules without bonds to the polymer molecules (hydration). At the same time, the aggressive agent can condense in pores and sink holes. The overall result is swelling independent of the diffusion rate that increases relatively slowly towards a state of saturation. Such swelling processes are reversible to a certain degree [243]. 

The diffusion of solvating media results in addition of diffused-in media molecules to polymer macromolecules due to intramolecular forces (in polar polymers such as PVC due to dipole-dipole interaction). The macromolecules are then virtually encased by media molecules. This increases the spacing between molecules, thus leading to reduced bond forces and increased macromolecular mobility. In turn, this allows even more molecules to penetrate into the intermediate spaces, resulting in a reduction in strength and an increase in elasticity the plastic becomes soft and under the effect of solvents decomposes completely [243]. 

All reversible processes of a physical nature can be reversed to some extent, i.e., the solvent molecules can be extracted by drying at elevated temperatures and/or reduced pressure so that the swelling recedes. Plastics treated in this manner often indicate only minor changes in their properties compared to their original condition. However, if the result is removal of plasticizers or other additives, properties may decrease. 

The increased macromolecule mobility facilitates relaxation of internal stresses in the plastic so that strength can temporarily increase the macromolecules may also become more easily oriented and in semi-crystalline plastics the processes associated with secondary crystallization can proceed more easily [86]. 

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A physically active medium produces a physical change in the polymer such as swelling or solubilization without affecting the chemical structure of the polymer. Changes caused by physically active media are reversible. Swelling and solubilization are produced by inactive solvents. The criteria for establishing solubility have already been discussed in Section 1.1. 

Different lasers use different materials as the active medium. The medium can be either solid, liquid, or gas, and there are advantages for each in the amount of energy that can be stored, ease of handling and storage, secondary safety hazards, cooling properties, and physical characteristics of the laser output. 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

Rather than physically removing or misaligning a mirror, which is a slow procedure, 2 Switching is accomplished by introducing large losses in the optical resonator during pumping. Essentially, a shutter is placed between the active medium and the HR resonator mirror. With this shutter closed, the HR mirror is blocked to prevent feedback, and thus laser... 

Magnesium, harder than alkali metals, is physically activated by superficial oxide fracturing.i An effective method also consists of transforming the metal into a labile soluble complex with anthracene which decomposes in the medium (Eq. 2). The formation and decomposition reactions are both accelerated by sonication.45 The very active metal slurry was used in Grignard syntheses. ... 

Physical ageing can result from the spatial reorganization of polymer chains or segments (relaxation of enthalpy, volume, orientation or stress crystallization etc.), transport phenomena (penetration of a solvent, migration of additives) and superficial phenomena (e.g. cracking in a tension-active medium). 

Depending on the nature of the solid and the medium and the conditions under which the interaction between the media and the solid takes place, the effects of the reversible physical-chemical action of the active medium can manifest themselves differently. Namely, they may be revealed as strength decrease, embrittlement, or the facilitation of plastic deformation (adsorption-induced plasticizing). The principal factors that influence the form and intensity of adsorption-induced processes may be subdivided into three main groups ... 

Some substances, such as water, produce both chemical and physical changes when they come in contact with polymers. For instance, water is absorbed by polycaprolactam up to 3.5% by weight of polymer at room temperature, producing swelling. This physical change becomes more important at the boiling temperature of water, where polycaprolactam absorbs up to 12% of water and some hydrolysis of the polyamide is produced, i.e., water becomes also a chemically active medium. 

The resistance of a plastic to physically active media can be roughly evaluated if the polarities of both partners are known. The polarities of a plastic and the acting medium can be evaluated on the basis of the cohesion energy density of the individual substances. These data are available both for plastics and for various com-... 

In homogeneous catalysis, the catalytically active species is dissolved in the reaction medium and is present uniformly throughout the system. However, with resin catalysis, the catalytically active groups are anchored to the matrix and in the solvent-resin system are located at the surface of and within the body of the resin bead only. Ion exchangers are, in fact, particulate active. species and when used as catalysts combine with the physical and mechanical benefits of heterogeneous catalysts (Pitochelli, 1980). 

In summary, it is the effective surface area of a drug particle that determines its dissolution rate. The effective surface area may be increased by physically reducing the particle size, by adding hydrophilic diluents to the final dosage form, or by adding surface-active agents to the dissolution medium or to the dosage form. 

Results from constant differential pressure filtration tests have been analyzed according to traditional filtration science techniques with some modifications to account for the cross-flow filter arrangement.11 Resistivity of the filter medium may vary over time due to the infiltration of the ultrafine catalyst particles within the media matrix. Flow resistance through the filter cake can be measured and correlated to changes in the activation procedure and to the chemical and physical properties of the catalyst particles. The clean medium permeability must be determined before the slurries are filtered. The general filtration equation or the Darcy equation for the clean medium is defined as... 

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